THS7319_14 Datasheet, PDF (22/34 Pages) Texas Instruments – 3-Channel, Very Low Power Video Amplifiers with EDTV Filters and 6-dB Gain

THS7319
SBOS468A – JUNE 2009 – REVISED JULY 2009.............................................................................................................................................................. www.ti.com
Another benefit of the THS7319 over a passive RLC
filter is the input and output impedance. With a
passive filter, the input impedance presented to the
DAC varies significantly, from 35 Ω to over 1.5 kΩ,
and may cause voltage variations over frequency.
The THS7319 input impedance is 2.4 MΩ, and only
the 2-pF input capacitance plus the PCB trace
capacitance impact the input impedance. As such,
the voltage variation appearing at the DAC output is
better controlled with a fixed termination resistor and
the high input impedance buffer of the THS7319.
On the output side of the filter, a passive filter again
has a large impedance variation over frequency. The
EIA770 specifications require the return loss to be at
least 25 dB over the video frequency range of use.
For a video system, this requirement implies that the
source impedance (which includes the source, series
resistor, and the filter) must be better than 75 Ω,
+9/–8 Ω. The THS7319 is an operational amplifier
that approximates an ideal voltage source, which is
desirable because the output impedance is very low
and can source and sink current. To properly match
the transmission line characteristic impedance of a
video line, a 75-Ω series resistor is placed on the
output. To minimize reflections and to maintain a
good return loss meeting EIA specifications, this
output impedance must maintain a 75-Ω impedance.
A wide impedance variation of a passive filter cannot
ensure this level of performance. On the other hand,
the THS7319 has approximately 1 Ω of output
impedance, or a return loss of 40 dB, at 11 MHz.
Thus, the system is matched significantly better with
a THS7319 compared to a passive filter.
One final benefit of the THS7319 over a passive filter
is power dissipation. A DAC driving a video line must
be able to drive a 37.5-Ω load: the receiver 75-Ω
resistor and the 75-Ω impedance matching resistor
next to the DAC to maintain the source impedance
requirement. This requirement forces the DAC to
drive at least 1.25 V_P (100% saturation CVBS)/37.5 Ω
= 33.3 mA. A DAC is a current-steering element, and
this amount of current flows internally to the DAC
even if the output is 0 V. Thus, power dissipation in
the DAC may be very high, especially when three
channels are being driven.
Using the THS7319 with a high input impedance can
reduce DAC power dissipation significantly. This
outcome is possible because the resistance that the
DAC drives can be substantially increased. It is
common to set this resistance in a DAC by a
current-setting resistor on the DAC itself. Thus, the
resistance can be 300 Ω or more, substantially
reducing the current drive demands from the DAC
and saving significant amounts of power. For
example, a 3.3-V, three-channel DAC dissipates
330 mW alone for the steering current capability
(three channels × 33.3 mA × 3.3 V) if it must drive a
37.5-Ω load. With a 300-Ω load, the DAC power
dissipation as a result of current steering current
would only be 41 mW (three channels × 4.16 mA ×
3.3 V), or over eight times lower power. For overall
system power, this scenario must also account for the
THS7319 power. The THS7319 only consumes
3.4 mA total quiescent current. The quiescent power
added is then 3.3 V × 3.4 mA = 11.2 mW. The total
system power is then 41 mW + 11 mW = 52 mW, or
a factor of six times lower power compared to the
DAC driving the line directly. Saving power by adding
the THS7319 in a system is easy to see and
accomplish, not to mention the added benefit of a
three-pole filter on each channel.
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